Method and apparatus for generation of fine particles

ABSTRACT

To supply microfine liquid droplets to a microscopic space for enabling micromachining and provide a method and an apparatus for forming the microfine liquid droplets, there is provided a method and an apparatus for generating liquid fine particles, comprising atomizing a liquid, fractionating the atomized liquid particles to form microfine liquid droplets by inertial fractionation and contacting the microfine liquid droplets with a heated carrier gas, thereby thermally drying the liquid particles to form finer particles.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method and an apparatus for thegeneration of liquid fine particles, more specifically, the presentinvention relates to a method and an apparatus for the generation ofliquid fine particles, which can generate microfine liquid particles ofa submicron order.

BACKGROUND ART

Recently, manufacture of sensors or actuators, with microscopicdimensions, and by using micromachining, is attracting attention.

For example, a semiconductor sensor with a three-dimensional structurehaving a cross-section shown in FIG. 1 has been proposed. In FIG. 1, forexample, a microscopic space 3 giving a beam interval of 10 μm and afloating height of 2 μm from the silicon chip 1 is created in a siliconchip to form a cantilever 2. This microscopic space 3 is created duringthe formation of a silicon substrate 4 by embedding a sacrificial oxidefilm layer 5 into the portion which becomes the space later, forming asensor structure on a silicon layer 6 above the sacrificial oxide filmlayer 5 by patterning, selectively oxidizing the silicon layer 6, andthen dissolving and removing the oxide pattern 7 between beams 2 and thesacrificial oxide film layer 5 in the portion under the beam 2. Toremove this oxide film layer, removal by a hydrofluoric acid solution isthe simplest and easiest method but when the oxide film layer becomes awinding microscopic space and is treated with a hydrofluoric acidsolution as a liquid, the liquid attaches to the narrow space during theremoval treatment or the drying of the treating solution and, by thesurface tension thereof, a movable beam is drawn to a fixed beam. Whendrying is performed when this force is applied, the movable beam sticksto the fixed beam in a stuck state and the product obtained cannot beused as a sensor. Therefore, in order to attain the removal while notimposing a load such as surface tension of liquid, use of a gaseoushydrofluoric acid (hydrofluoric acid anhydride) is necessary. However,the reaction proceeds at an extremely low rate with only a hydrofluoricacid anhydride and therefore, water such as pure water or an alcoholmust to be mixed in the form of a vapor. More specifically, as shown inFIG. 3, a silicon chip 1 comprising a sacrificial oxide film layer 5 isplaced in a treatment chamber 11 and a hydrofluoric acid anhydride 13and water vapor 14 are introduced into the treatment chamber 11 underheating by a heating device 12 and mixed in the treatment chamber 11,thereby removing the sacrificial oxide film layer 5.

However, the method of introducing a hydrofluoric acid anhydride 13 anda water vapor 14 into the above-described treatment chamber 11 andmixing them in the treatment chamber 11 has a problem in that thebalance between the hydrofluoric acid anhydride and the water vapor isdifficult to control and, in the case of processing a complicatedstructure, the microscopic space is filled with excess water and, afterthe removal of the sacrificial oxide film layer, the structurecollapses.

If the amount of water vapor is reduced not to generate excess water,this processing method is difficult to use in practice because, forexample, the reaction rate seriously decreases and the process becomesextremely low in the productivity.

Under these circumstances in conventional techniques, an object of thepresent invention is to supply microfine liquid droplets to theabove-described microscopic space and, to realize this, to provide amethod and an apparatus for forming microfine liquid droplets.

SUMMARY OF THE INVENTION

To attain the above-described object, the present invention provides thefollowings.

(1) A method for generating liquid fine particles comprising atomizing aliquid to form atomized liquid particles, fractionating microfine liquiddroplets from said atomized liquid particles by inertial fractionation,and contacting said microfine liquid droplets with a heated carrier gas,to thereby thermally dry said microfine liquid droplets to form liquiddroplets which are finer than said microfine liquid droplets.

(2) The method for generating liquid fine particles as described in (1)above, wherein in said fractionating, microfine liquid droplets having adiameter of 10 μm or less are fractionated by inertial fractionationfrom atomized liquid particles which are generated by the atomizationand include liquid particles having a diameter exceeding 10 μm, and saidfractionated microfine liquid particles having a diameter of 10 μm orless are then thermally dried to form further finer liquid particleshaving a diameter of 1 μm or less.

(3) The method for generating liquid fine particles as described in (1)and (2) above, wherein the liquid is an aqueous solution and theinertial fractionation is fractionation utilizing the difference incollision energy of atomized liquid particles against a wall.

(4) The method for generating liquid fine particles as described in (1)to (3) above, wherein the inertial fractionation is performed twice ormore.

(5) The method for generating liquid fine particles as described in (1)to (4) above, wherein the atomization of the liquid using an injectiongas is performed in a liquid tank to form atomized liquid particles, theinertial separation is performed by colliding said atomized liquidparticles against the inner wall of the liquid tank, and thereby coarseliquid droplets in said collided atomized liquid particles arere-circulated to the liquid tank, by which the injection gas used isdissolved in a liquid in the liquid tank and, at jetting for saidatomization, the atomized liquid particles are decompressed to causeexpansion and bursting of the injection gas dissolved in the atomizedliquid to accelerate the formation of fine particles of the atomizedliquid.

(6) The method for generating liquid fine particles as described in (1)to (5) above, wherein the liquid is an etching solution and the liquidfine particles are used for micromachining.

(7) The method for generating liquid fine particles as described in (1)to (6) above, wherein the liquid is an aqueous hydrofluoric acidsolution and the liquid fine particles are used for etching a siliconoxide film.

(8) An apparatus for generating liquid fine particles, comprising meansof atomizing a liquid, means of inertially fractionating the resultingatomized liquid particles to obtain microfine liquid droplets, and meansof drying the obtained microfine liquid droplets with a heated carriergas to form finer liquid particles.

(9) The apparatus for generating liquid fine particles as described in(8) above, wherein the atomizing means has a constitution that aninjection gas is jetted in a liquid tank to cause suction and jetting ofthe liquid in the liquid tank and the atomized liquid particles formedby the jetting are collided against the inner wall of the liquid tank,thereby performing inertial fractionation.

(10) The apparatus for generating liquid fine particles as described in(9) above, wherein a nozzle is provided at the top of the liquid tankand microfine liquid droplets inertially fractionated by the collisionagainst the inner wall of the liquid tank are passed through the nozzleand accelerated to collide against a wall present above the nozzle,thereby again performing the inertial fractionation.

(11) The apparatus for generating liquid fine particles as described in(8) to (10) above, wherein a heated carrier gas is fed into thetransportation path of the inertially separated microfine liquiddroplets to dry the microfine liquid droplets and form finer liquidparticles.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a partial cross-sectional view of a semiconductor sensor.

FIG. 2 is a cross-sectional view of a silicon substrate having asacrificial oxide film layer for forming the semiconductor sensor ofFIG. 1.

FIG. 3 is a view showing a treatment chamber in conventionalmicroetching processes.

FIG. 4 is a view showing one example of a semiconductor sensor.

FIG. 5 is a schematic view for explaining a working example of themethod and the apparatus for forming ultrafine liquid droplets accordingto the present invention.

FIG. 6 is a view showing the principle of inertial fractionation.

MODE FOR CARRYING OUT THE INVENTION AND EXAMPLES

By noting the fact that the oxide film layer is stably removed by ahydrofluoric acid solution at a highest speed as described in theBackground Art, the present invention provides a method for producingultrafine liquid droplet particles capable of being directly fed intothe microscopic space unlike conventional techniques where ahydrofluoric acid solution is prepared by mixing a gas and a watervapor, and, thereby, solving the problems in conventional techniques.

The reason why water or an alcohol needs to be mixed with thehydrofluoric acid in use for the etching of a silicon oxide film is thatwhen water or an alcohol is present together with the hydrofluoric acid(here, when water is present) the following reaction takes place (asimilar reaction takes place also when an alcohol, for example, anethanol is present):2HF (g)+H₂O (aq)→2HF₂ ⁻+2H⁺+H₂O (aq)to produce F ion and, due to the presence of this F ion, the ollowingreaction occurs:SiO₂+2HF₂ ⁻+2H⁺→SiF₄+2H₂Owhereby etching of SiO₂ readily proceeds, however, when H₂O or analcohol is not present, the F ion is not produced and SiO₂ must beetched only with anhydrous HF and the etching rate is unlimitedly low.Therefore, introduction of water is inevitable at present.

FIG. 4 shows an example of a semiconductor sensor. In the Figure, 21 isa silicon substrate, 22 is an embedded oxide film, 23 is a siliconlayer, 24 is a movable electrode, 25 is a fixed electrode, 26 is a beam,27 is a plumb, 28 is an anchor and 29 is a pad.

The present invention provides a method for forming ultrafine particlesof a hydrofluoric acid solution for performing the microfine processingof a microdevice as shown in FIG. 1 or FIG. 4.

FIG. 5 shows the state where the method of the present invention ispracticed. A hydrofluoric acid solution 32 is charged in a chemical tankand, for example, a nitrogen gas 34 is fed into an atomization nozzle 33to atomize the hydrofluoric acid solution 32. The atomized liquidparticles 35 of the hydrofluoric acid solution contain particles havinga diameter of 20 μm or more. These atomized liquid particles 35 arecollided against the inner wall 36 of the chemical tank 31 and, as aresult, atomized liquid particles 35 having a large diameter attach tothe wall 36 due to their large inertial energy, whereas atomizedmicrofine liquid droplets can be reflected from the wall due to theirsmall inertial energy. For example, microfine liquid droplets 37 havinga diameter of less than 10 μm can remain above the surface of the liquid32 in the tank 31.

FIG. 6 shows the principle of inertial fractionation of liquid dropparticles. In the case where the liquid droplet jetted out from theejection nozzle 51 is a large particle 52, the liquid droplet 52 iscrushed onto colliding against a wall 53 due to its large inertialenergy, attaches 54 to the wall 53 and is not repelled. On the otherhand, in the case where the liquid droplet jetted out from the ejectionnozzle 51 is a small particle 55, the liquid droplet 55 is not crushedon colliding against the wall 53 due to its small inertial energy butcan be repelled 56 from the wall and can float. As schematically shownin the left of FIG. 6, when the particle size is larger than a certainvalue, the liquid droplet is crushed and not repelled, whereas when theparticle size is smaller than a certain value, the liquid droplet is notcrushed and is repelled. By making use of this property, liquid dropletparticles having a small particle size can be fractionated. The size ofthe liquid droplet fractionated can be changed to a certain extent bycontrolling the size of the liquid droplet atomized and the speed incolliding against wall.

In the atomizing apparatus shown in FIG. 5, an atomization nozzle isprovided in the chemical tank and therefore, an ejection gas (nitrogengas) is dissolved in the chemical (hydrofluoric acid solution) to asaturated concentration. When this hydrofluoric acid solution havingdissolved therein a nitrogen gas to a saturated concentration isatomized, the hydrofluoric acid solution is decompressed at theatomization and, as a result, the nitrogen gas dissolved therein expandsand acts to tear apart the liquid droplet and this provides an effectthat fine particles can be more effectively formed. Furthermore, theparticle size can be controlled independently of the concentration ofliquid droplet chemical (which is adjusted by the concentration ofsolution) and this provides an effect that the conditions of varioustreatments using the liquid droplets can be established over a widerange. In addition, the liquid is allowed to act directly and thisprovides an effect that a treating rate far higher than in aconventional gas-type treatment can be realized.

The microfine liquid droplets 37 of less than 10 μm may be sent to thedrying step but, in FIG. 5, a nozzle part 38 is further formed at thetop of the chemical tank 31, whereby the microfine liquid droplets 37which are light and therefore, ascend due to pressure of the atomizinggas are accelerated through the nozzle part 38 to collide against theinner wall 40 of a buffering chamber 39 and are again sorted by inertialenergy into finer particles. By passing through this process, the liquidparticles can be made and sorted even into a size of a few μm or less.

The microfine liquid droplets 41 thus sorted into a size of a few μm orless are supplied to a treatment chamber 46 but in the supply tube 42are mixed with a nitrogen gas 44 heated through a heating chamber 43,whereby the microfine liquid droplets 41 of a few μm are heated anddried and the components are evaporated according to the degree ofheating, as a result, individual microfine liquid droplets are formedinto finer particles and ultrafine liquid droplets 45 are produced. Theultrafine liquid droplets 45 may have a size of less than 1 μm. Byselecting the conditions for inertial fractionation and dry processingor depending on the kind of liquid, further finer liquid droplets can bealso obtained, for example, ultrafine liquid droplets of 0.4 μm or less.

When liquid droplets of the hydrofluoric acid solution thus formed intoultrafine particles of less than 1 μm are introduced into a treatmentchamber 46 and used for the etching to remove an oxide film layer 48 ofa silicon chip, as the diameter of the liquid droplets is sufficientlysmall, the liquid droplets, even if attached to a beam 49, do not act todraw a movable beam and a fixed beam, which are formed with a distanceof 10 μm therebetween, to each other by the surface tension of theliquid. Moreover, the hydrofluoric acid solution contains water andtherefore, the etching of silicon can proceed at a desired high speed.FIG. 5 shows ultrafine liquid droplets 45 drawn in the oxide film layer48 but this is the state where etching of the oxide film proceeds toform a vacancy and, thereafter, the ultrafine liquid droplets 45 intrudeinto the vacancy. Needless to say, ultrafine liquid droplets 45 are notpresent there when an oxide film exists.

In the foregoing pages, the case of forming microfine particles of ahydrofluoric acid solution by using a nitrogen gas is described,however, the kind of the liquid is not limited and as can be understood,not only a solution but also one kind of liquid can be formed intoliquid droplets and into microfine particles and furthermore, theatomization or heat-drying treatment can be performed by using a gasother than nitrogen.

The concentration of hydrofluoric acid solution used, conditions foratomization, conditions for collision against wall of chemical tank orbuffer chamber, and conditions for drying under heat can be freelyselected according to the kind of treatment. For example, in order toetch a silicon oxide film shown in FIG. 1, an aqueous 49% hydrofluoricacid solution was used as a raw material and ultrafine liquid dropletsof less than 1 μm were formed as described above. At this time, theetching of the silicon oxide film was performed in a treatment chamberat 40° C. but in the heat-drying treatment with nitrogen gas at theprevious stage, a nitrogen gas heated at 60 to 90° C. was used. Theheat-drying treatment is apparently affected by the flow rate of heatinggas as well as the temperature and therefore, the temperature and theflow rate should be appropriately selected according to the need. In theExample, the treatment of removing the oxide film was suitably performedwhen 20 to 40% of the hydrofluoric acid solution was evaporated with aheating gas at a flow rate of 8 to 12 L/min. When the hydrofluoric acidsolution was insufficiently evaporated, sticking of beams occurred.

An oxide film of a semiconductor sensor was etched by using ultrafineliquid droplets of hydrofluoric acid solution prepared in Example of thepresent invention, as a result, sticking of beams did not occur evenwhen the oxide film was etched at an etching rate of 15 to 25 nm/min.

On the other hand, a method of etching an oxide film of completely thesame semiconductor sensor by using a liquid hydrofluoric acid, a methodof introducing dry hydrofluoric acid anhydride and water vapor into atreatment chamber and mixing them there, and a method of performing theetching with a vapor phase containing hydrogen fluoride gas, a slightamount of water vapor and nitrogen described in Kokai No. 5-275401 werepracticed and compared. In the method of using a liquid hydrofluoricacid, sticking could not be prevented. In the method of introducing dryhydrofluoric acid anhydride and water vapor into a treatment chamber andmixing them there, the etching rate was only from 0.2 to 2 nm/min underconditions that did not cause sticking. In the method described in KokaiNo. 5-275401, the etching rate was from 2 to 4 nm/min under theconditions of not causing sticking. Accordingly, it is verified that inthe method of the present invention, an etching rate as high as about 5to 10 times can be realized under conditions of not generating stickingof beams as compared with the method described in Kokai No. 5-275401.

The etching rate shown here is determined for a specific semiconductorsensor and varies when the setting conditions are changed but theusefulness of the present invention is not lost.

According to the present invention, a method and an apparatus forforming liquid droplet fine particles into ultrafine particles of 1 μmor less are provided. By using a treating solution after forming it intomicrofine particles, wet processing free of defects can be attained evenwhen the microfine particles are used for micromachining.

1. A method for generating liquid fine particles comprising atomizing aliquid to form atomized liquid particles, fractionating microfine liquiddroplets from said atomized liquid particles by inertial fractionation,and contacting said microfine liquid droplets with a heated carrier gas,to thereby thermally dry said microfine liquid droplets to form liquiddroplets which are finer than said microfine liquid droplets.
 2. Themethod for generating liquid fine particles as claimed in claim 1,wherein in said fractionating, microfine liquid droplets having adiameter of 10 μm or less are fractionated by inertial fractionationfrom atomized liquid particles which are generated by the atomizationand include liquid particles having a diameter exceeding 10 μm, and saidfractionated microfine liquid particles having a diameter of 10 μm orless are then thermally dried to form further finer liquid particleshaving a diameter of 1 μm or less.
 3. The method for generating liquidfine particles as claimed in claim 1, wherein said liquid is an aqueoussolution and said inertial fractionation is fractionation utilizing thedifference in collision energy of atomized liquid particles against awall.
 4. The method for generating liquid fine particles as claimed inclaim 1, wherein said inertial fractionation is performed twice or more.5. The method for generating liquid fine particles as claimed in claim1, wherein said atomization of the liquid using an injection gas isperformed in a liquid tank to form atomized liquid particles, theinertial separation is performed by colliding said atomized liquidparticles against the inner wall of the liquid tank, and thereby coarseliquid droplets in said collided atomized liquid particles arere-circulated to the liquid tank, whereby the injection gas used isdissolved in a liquid in the liquid tank and, at jetting for saidatomization, the atomized liquid particles are decompressed to causeexpansion and bursting of the injection gas dissolved in the atomizedliquid and to accelerate the formation of fine particles of the atomizedliquid.
 6. The method for generating liquid fine particles as claimed inclaim 1, wherein said liquid is an etching solution and said liquid fineparticles are used for micromachining.
 7. The method for generatingliquid fine particles as claimed in claim 1, wherein said liquid is anaqueous hydrofluoric acid solution and said liquid fine particles areused for etching a silicon oxide film.